Hardcopy output devices, such as printers and fax machines, frequently use inkjet technology to form text and images on print media such as paper, transparencies, and textiles. Hardcopy devices of this sort are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). A printhead in these output devices is connected to an ink supply and contains an arrangement of nozzles and a control mechanism which allows ink drops from the ink supply to be controllably ejected from each individual nozzle onto the media. Color inkjet printing devices frequently contain several printheads, typically four to six. The printheads generally are mounted side-by-side in a carriage which scans them back and forth within the printer in a forward and a rearward direction above the media during printing such that the individual printheads move sequentially over a given location on the medium. As the printhead is moved relative to the print medium, a controller selectively activates individual printing elements in the printhead to eject ink droplets through the nozzles and deposit them onto the print medium. Since the printheads are generally much smaller than the image to be printed, the image is divided into regions of appropriate size, called swaths, each of which can be printed in a forward or rearward scan of the carriage. The layout of the printhead nozzles determines the size of the media swath that can be printed during a scan. The printer also has a print medium advance mechanism which moves the media relative to the printheads so that, by combining the scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media. The controller typically orchestrates the scanning, media advance, and ink drop ejection operations, including such printing aspects as how many scans are required to fully print a section of the media, how much the media is advanced between scans, and which portions of the image are printed during which scans. Further information as to the basics of inkjet printing technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
A color inkjet printing system typically uses several different color ink supplies, each fluidically connected to one of the printheads, to produce color print output. A typical set of four color inks includes cyan, magenta, yellow, and black inks. During printing, drops of different ones of these inks may be deposited in the same or adjacent pixel locations to form a range of colors as perceived by the human eye.
The fastest way to print a swath is to scan in one direction while ejecting all the ink drops associated with the image to be printed in the swath, advance the media the height of the swath, then scan in the opposite direction while ejecting ink drops as before. This is known as single-pass bidirectional printing. It is “single-pass” because the printheads pass over each area of the media only one time. It is “bidirectional” because drops are fired while the printheads are traveling in both the forward and rearward scan directions. However, single-pass bidirectional printing often suffers from reduced image quality that is particularly noticeable when a region of a particular color is partially printed in the forward scanning direction and partially printed in the rearward scanning direction. This image quality degradation occurs because the different color printheads are located in the carriage in a fixed sequence, thus reversing the order in which the different color ink drops will be deposited in a given location on the print medium in each scanning direction. Since the order in which different color inks are deposited on the print medium often slightly changes the hue or shade of the color as perceived by a human observer due to interactions between the ink and the media, image quality can suffer. This image quality defect is usually referred to as “bidirectional hue shift”.
Accordingly, it would be highly desirable to have a new and improved inkjet printing system and printing method that reduce bidirectional hue shift without compromising throughput and without requiring extra cost or complexity in the printing system. Such a system and method may solve other problems as well.